Building a large 3d printer from UM

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I am new in this forum and I am wandering if not all UM enthusiast has been thinking to build a larger printer after having the UM original. Having a mechanical engineer background, I was thinking to design and build a 900 x 900 x 900 useful volume 3d printer for a special project I have in mind since years. The mechanical part of that machine present several challenges but I am confident they can be handled somehow. My problem is the modifications needed in the electronics and control program for the step motors. By now I have made only sketches and a brief study of the structure, the x, y and z axis, but before to go further in details I would like to have your expert opinion concerning the following topics:

1. The printing bed would be a large piece of glass, 1000 x 1000 x 12 mm approximately, having a mass of about 30 kg, let's say 33 kg with an special support. I am planning to use two lead screws of 10 mm with a pitch of 3 mm and four 20 mm diameter rods as guiding. In this way two step motors Nema 17 could handle the weight with a torque of about 0.145 Nm each, lifting the bed and 0.043 Nm going down. (Holding torque is 0.44 Nm according to the motor specifications).

Would I need to change the step motor driver ? Or the original driver can handle the current since most the time gravity help me a lot and in any case I can lift by hand the printed bed once the printed model is finished. Very short time is used to send the printing bed to home during its level adjustment, so hopefully the driver can support that current briefly.

Another question, is it absolutely necessary to link mechanically the step motors to synchronize them or the electronics are in charge of this task. Is it safe enough ?

2. The pulley for driving the x and y axis would be bigger than the original. This creates problems with the speed and exact positioning of the printer head. For example, using a preliminary arrangement, if the model ask for a 100 mm displacement, the print head will travel about 146 mm and at higher speed than the program "thinks", this would force me to change the feeding rate of the PLA filament.

Although I can modify the gear ratio in the extruder, is there another way to modify the parameters in the control program to adjust the accuracy of the movement and the feeding rate as needed? How to do it ? I am completely new to such things, but I do not care if I have to study something new, so some hints would be appreciated.

3. Printing head is moving on a gantry rail using a very large toothed belt fixed to the structure as well as the driving motor. This increase the weight of the moving parts compared to the original arrangement so I think I should reduce the maximum travel speed about 150 mm/s despite of the longer printing time. On the other hand this increase in weight is rewarded with a gantry deflection of about 0.04 mm in worst case. I have to study more in detail this subject.

What would be the best toothed belt profile to use? By now I have study this subject using a AT5 profile but probably for this application it is not the best option.

4. Other points that I have to take in consideration?

Hopping to hear from you soon I wish you Merry Christmas and the best for the incoming new year

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For this size I think you are better off going for a H gantry with proper linear rails and bearings.

The program changes you are talking about are not major and can be easily done.

If you use the um original board the I believe the stepper drivers plug in. Unlike the UM0+ and UM2. These can be changed as there are topics of people changing them to a more quiet one. Maybe you can find one to handle the higher current or at least see the specs on what they can handle.

Just consider that the bigger the base the more prone to warping.

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Yes Dim3nsioneer, the aluminum frame would be strong and straightness should be checked frequently, I am wandering how to do it easily and properly, may be using dial indicators some how... I will have a look on BigRep. Since this printer is not intended for commercial affaires, the cost is not a priority, nevertheless I am looking for cheaper solutions but effective ones.

Labern... The gantry I am planning to use is an aluminum profile de 30 x 30 mm with a rail and guide for the printing head. I am trying to have as less weight as possible on that rail avoiding to place the step motor in the Y axis. Motor will be fixed to the frame, so it is not moving with the printing head. Concerning the changes in the program, I am happy to hear that those changes are not difficult to make but I do not know how to proceed. Calculating the parameters is not a problem but change them is another story. Any hint on that ?

I am not able to deal with electronic circuits, so changing the drivers on the circuit board is not my business. Probably it is better to leave the printing bed fixed and move the x and y axis frame up and down, in this way I can use the original stepper driver reducing the current in the motor drastically.

We'll see...

I am aware of the wrapping problem with large printing bed. Is the heating bed the only solution to that problem? Is it really a solution? Sorry I have not experience in 3D printing but I am eager to learn.

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Many thanks for taking your time in reply, I have appreciated it very much. Yeah... the printing time will be long but must be a "pause" or something like that in CURA, or let the machine run until filament is almost finished and continue with a new roll of PLA after pause.

Peetersm, thank you for the input concerning the adjust in the firmware. I'll study it in detail soon. This is exactly what I have in mind when I ask about the changing in parameters because with the new set-up it simply won't work. Tanks again.

How about heating bed is it absolutely necessary or I can print a sacrificial layer to let it wrap and cut away after printing is finished ? Sorry but I am really new in 3D printing.

Rafael

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I have several UMO+ print in PLA only and am not a fan of heated beds, due to the known issues of edges curling up, wrinkled lower layers, longer cool down times to start another job, etc. I use blue tape on glass. (ok...well I do use the heated bed in the winter, and only to 26C because my basement is cool)

Not sure you'll need a heated bed, but I will bow to those who know more about large printers..

actually thinking about it, when I print a very rigid and large surface area on bed object it is pretty hard to get it off the bed...YOU may want a heated bed because removing an object from that large of the surface might be difficult without the heated bed's "shrinking and popping off" feature. ok now I'm not sure what to tell you.

I just know that when I print an object that is large and covers most of the bed it can be very difficult to remove it from the glass.

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Thank you Peetersm. I will take into account your comments. I am thinking to use a commercial flat heater that covers most of the glass surface and make some tests. Or probably to enclose the large 3D printer in a very small room (in the basement) with provisions for cooling down and temperature control.

Another question if you don't mind, printing a large object, more than 1 m height for example, what would be the coefficient of contraction that I have to add in the model in order to get the proper dimension of the object when cooled down ?

Thanks again

Rafael

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This is so dependent on so many things. Just draw it at the correct dimensions and alter your printing temp or speed to get the correct outer dimensions, small Inner circles can then be measured and altered in your drawings.

Given a fixed speed, altering the nozzle temp can change the line width slightly. If you have the correct E steps them this can give slight adjustments to make you part more accurate. But room temp can also come in to play and needs to be kept at a constant. But all this depends on how accurate you need to be.

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With a much larger bed power becomes an issue. The current UM2 power supply is barely strong enough. If you have a larger heated bed, more power is required (so you probably need a separate power supply)

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Yes I am aware of that. For this reason I am thinking to let the heated bed fixed and move the x-y axis frame up and down along the z axis. This means that, contrary I have stated above, now the gravity is not helping too much except to counteract the back lash a little bit.

On the other hand I am still thinking to use two step motors to drive the z axis using belts and a reduction gear of 1:20 (double reduction, 1:4 and 1:5) in order to compensate the replacement of the lead screw. Using belts, even in the reduction box, I try to be more efficient, since belt driving is better than screw in that point. Probably I would have more back lash with this arrangement but I can live with that lack of precision.

A question that is still in the air is : should I synchronize mechanically the step motors driving the z axis ? or electronically is enough ?

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I see what you mean. Yes the power supply for the heated bed will be separated, I was concern more in the power to move the entire frame with the x and y axis including the printing head. Lead screws have an efficiency of about 30% so I am thinking to use two toothed belts to move the frame along the z axis guided by four 30 mm diameter hardened steel rods, in this way I can retain the same motors with much more weight to move.

Whenever I have my design complete I'll post it to have your comments and remarks.

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Yes, I was thinking something like that. After your suggestion I've made a quick research in Internet and found that is exactly what I have in mind. Weight is less and motors can be fixed in the structure so they do not have to move back and forth quick which cause a lot of forces during acceleration and deceleration of the printing head. I like this particular point very much.

Also I've noticed that some of the arrangement presented in my search had two step motors driving the z axis, without apparent mechanical synchronization between them. So it is very promising in case I need 2 motors. W'ill see...

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